installer: Add "Kernel" page to select the Hurd.

This adds a "Kernel" page to the installer with the option to (cross-) install
the Hurd, if applicable (only available on x86 machines for now).

* gnu/installer/newt.scm (kernel-page): New procedure.
(newt-installer)[kernel-page]: New field.
* gnu/installer/kernel.scm,
gnu/installer/newt/kernel.scm: New files.
* gnu/local.mk (INSTALLER_MODULES): Add them.
* gnu/installer.scm (installer-steps): Use them to select kernel if
applicable.
* gnu/installer/newt/partition.scm (run-label-page): Default to "msdos" when
instaling the Hurd.
(run-fs-type-page): Add ext2 for the hurd.
(run-partitioning-page-partition): Remove `entire-encrypted' option when
installing the Hurd.
* gnu/installer/services.scm (system-services->configuration): Cater for the
Hurd with %base-services/hurd, and with %base-packages/hurd that must always
be set.
(%system-services): Change to procedure.  When installing the the Hurd, do not
recommend `ntp-service-type' and USE `openssh-sans-x' package for
`openssh-service-type'.
(system-service-none): New variable.
* gnu/installer/newt/services.scm (run-network-management-page): Include it
when installing the Hurd.
(run-desktop-environments-cbt-page): When installing the Hurd, recommend to
not select any desktop enviroment.  Update users.
* gnu/installer/parted.scm (efi-installation?): Return #f when installing for
the Hurd.
(create-ext2-file-system): New procedure.
(user-fs-type-name, user-fs-type->mount-type, partition-filesystem-user-type,
format-user-partitions): Support `ext2'.
(<user-partition> partition->user-partition): Use `ext2' when installing the
Hurd.
(auto-partition!): Likewise.  No swap partition when installing the Hurd.
* gnu/installer/final.scm (install-system): Cater for cross installation of
the Hurd.
(bootloader-configuration): Use `grub-minimal-bootloader' when installing the
Hurd.
(user-partition-missing-modules): Cater for empty user-partitions.
(initrd-configuration, user-partitions->configuration): Cater for the Hurd.
* gnu/installer/steps.scm (format-configuration,
configuration->file): Cater for the Hurd.
* gnu/system/hurd.scm (%desktop-services/hurd): New variable.
* gnu/installer/tests.scm (choose-kernel): New procedure.
* gnu/tests/install.scm (gui-test-program): Use it.

Change-Id: Ifafb27b8a2f933944c77223a27ec151757237e36

gnu/system/hurd.scm

@@ -48,6 +48,7 @@
   #:export (%base-packages/hurd
             %base-services/hurd
             %base-services+qemu-networking/hurd
+            %desktop-services/hurd
             %hurd-default-operating-system
             %hurd-default-operating-system-kernel
             %setuid-programs/hurd))
@@ -107,6 +108,8 @@
                   %qemu-static-networking))
    %base-services/hurd))
 
+(define %desktop-services/hurd %base-services/hurd)
+
 (define %setuid-programs/hurd
   ;; Default set of setuid-root programs.
   (map file-like->setuid-program